As video becomes a more critical tool in physical security applications, it is important to understand the technology from a transmission standpoint. Video is an extremely useful tool for detecting and investigating incidents. However, if the video quality is not good enough to detect or determine the details of an incident, the system is a poor investment.
The transmission infrastructure is often the cause of quality problems in video, but these problems are preventable with the proper system design and installation. A key to proper system design is a well-designed transmission infrastructure.
Challenges to Transmission
Video signals must be transmitted some distance from cameras to recorders and display monitors, and they must retain the same quality that was developed at the source. This is a major challenge due to a number of common factors.
Bandwidth Limitations. Video is a complex, broad-bandwidth signal in its basic form. Video transmissions contain complex signals whose components are near 0 Hz at the low end of the video frequency spectrum and as high as 10 MHz for highly detailed information. Anything that restricts this bandwidth will affect the image quality. Low-frequency response is as critical as high-frequency response.
Frame Rate. The video signal contains a real-time stream of images running at 60 fields/second (the U.S. standard for live video). In some restricted bandwidth channels, the signal is modified to deliver less than real time. In slow-scan systems this is referred to as refresh rate.
Interference from External Sources. The video signal is susceptible to electrical magnetic interference (EMI). High-frequency interference will affect the detail components of the image, while low-frequency interference, such as power lines, will affect the video signal with rolling noise bars.
Ground Loops. Ground loops are a common problem in video signal transmission. Ground loops occur when different ground potentials exist throughout the infrastructure. They create the same type of visual interference as power lines.
Latency. Latency defines the time it takes the video signal to get from the source to the destination. Many of the transmission methods today cause significant latency, or delay. This becomes a serious problem when an operator is trying to operate a motorized camera (PTZ or dome). It is easy to overshoot the target if the latency is significant.
Security. The video signal may be intercepted or extracted by unauthorized people.
S/N Ratio. An overlooked issue in transporting video is the noise caused by interference or other sources during transmission. Noise from electronic processes, known as white noise, can affect the video image as speckles randomly dispersed over the scene. (Most of us have seen this in home videos with poor lighting.)
Noise mixed into a video signal can have a serious effect on the digitization of the signal for compression. The digitizer cannot distinguish between noise and valid video information and thus tries to compress the noise. The noise makes more work for the digitizer by appearing as picture details.
Cost. The transmission infrastructure is often the most expensive part of the video system. Scrimping in cost on the transmission infrastructure can result in unusable video, creating a system that doesn’t meet the security objectives.
When you budget for a transmission system, consider the following issues:
• Cost of the cable, wire or fiber optics medium. Plenum cable requirements should be considered in this phase of budgeting.
• Cost of the labor to pull the cabling.
• Cost of transmitters and receivers.
• Cost of support equipment such as video switchers, digital amplifiers (DAs) and video (i.e. analog) amplifiers (VAs).
Several transmission media and technologies can be used to transmit analog and digital video, and each deals with the above challenges more or less well.